replace URI.decode

Continuation of a3177072dfd829adb237904ce9583c461f258112
use method from WEBrick::HTTPUtils

This was initially taken from WEBrick and modified.
add Utils.uri_unescape
add rbx stdlib for tests
Merge pull request #333 from burns/321_cont

Continue URI.decode removal
fix for 1.8 www form encode

closes #334
remove deprecated usage
add Excon::Request as a subclass of Hash